A morphometric comparison was performed on specimens of blue catfish (Ictalurus furcatus) from northern (Lower Río Bravo) and southern (Chiapas) México in order to identify diagnostic characters that allow their discrimination. The discriminant function analysis determined three characters to be highly diagnostic to separate the two groups of specimens: the southern group [SG] has a shorter anal base (mean  =  3.4 times in standard length [SL], range  =  3.1 to 3.7) vs northern group [NG] (mean  =  2.9 times in SL, range  =  2.7 to 3.1), a lesser head width (mean  =  6.0 times in SL, range 5.1 to 6.8) vs NG (mean  =  5.6 times, range  =  5.3 to 6.0), and a lower number of anal rays (mean  =  26, range  =  24 to 28) vs NG (mean  =  31, range  =  29 to 34). Additionally, 14 other characters were also different (P < 0.01) between both groups. All these characters support the taxonomic validation of Ictalurus meridionalis (Günther 1864) for the individuals of SG that are currently included in I. furcatus Lesueur. Studies on comparative osteology and molecular genetics of both forms are needed for the clarification of their taxonomic status.